This invention relates to a system for preventing unauthorized operation of a vehicle having a passive entry/start system, and in particular one that uses a bi-directional radio frequency, or RF-RF, signal.
Modern vehicles are becoming equipped with passive entry/start systems whereby an authorized vehicle operator carries an electronic key or card that contains electronic identifying information. The entry system of the vehicle responds accordingly when the presence of the electronic identifying information, or ID device, is detected. Essentially, these passive systems do not require actuation of any lock, nor the use of any key. Instead, when the operator attempts to pull the door handle open, the vehicle interrogates the area around the vehicle to determine whether a valid ID device is detected. If so, then the door is opened, the ignition started, etc.
Typically, these passive entry/start systems employ a low frequency-radio frequency or LF-RF signal system. That is, the transmission sent from the vehicle to the ID device uses a low frequency (LF) signal and the transmission from the ID device to the vehicle is sent using a radio frequency (RF) signal. The LF signal is directional, and a distance limited signal.
While the LF-RF signal systems perform well, they are expensive in comparison with RF-RF signal systems considered for use in passive entry/start systems. A RF-RF signal system can be less expensive because there are fewer antennas required than the LF-RF signal system. The LF-RF signal system usually needs antennas at each entry point, such as doors and the trunk, and also in the interior for starting. Further, the RF-RF signal system uses a less expensive type of antenna.
The use of a RF-RF signal system in passive entry/start systems, however, presents a challenge in that the signal from the vehicle to the ID device is neither directional, nor significantly distance limited. This can create problems in which a thief can activate the system (by pulling on the door handle, closing a switch, mechanical or capacitive sensing, etc.) and gain unauthorized access to the vehicle when the ID device is left within range (eg: ID device in house and vehicle parked close to house) or when the ID device is left within the vehicle. If the ID device is within range, the thief could enter the car, start the car, and drive away. It would therefore be desirable to provide a passive entry/start system for use in vehicles with a less cost prohibitive bi-directional radio frequency signal that also would not be subject to unauthorized vehicle access and operation.
In a disclosed embodiment of this invention, a method is devised to prevent unauthorized access to a vehicle equipped with a bi-directional, signal passive entry/start system. The method comprises a system to measure the signal strength level received by the vehicle""s passive entry receiver from the driver""s ID device and compare the strength to a prior received signal.
At a time after the vehicle is turned off and locked, the signal strength level is measured. This signal strength is used as a threshold value. The signal strength level is stored, and may then be measured periodically. When a request occurs for a change in vehicle status, the vehicle""s passive entry control unit compares a new measured signal strength level to the threshold to make a prediction on whether a current request is authorized.
These and other features of the present invention can be understood from the following specification and drawings, the following of which is a brief description.